This invention relates to a novel cyclic octapeptide capable of intramolecularly binding a specific metal ion such as calcium ion or barium ion and consequently forming a complex and transporting, concentrating, and separating the ion, to a method for the production of the octapeptide, to an ion extracting/separating agent using the octapeptide as a principal component, and to a method for the extraction of ions by the use of the octapeptide. As concerns the industrial sectors in which this invention can be applied, it is useful in the analytical chemical industry and inorganic and organic chemical industry for analysis and separation of the aforementioned metal ions, in the synthetic chemical industry for catalysis of interphase transfer of the aforementioned metal ions, and in the medical and welfare fields for supply of the aforementioned metal ions where their presence is beneficial or for removal of the aforementioned metal ions where their presence is detrimental.
Heretofore, synthetic crown compounds and synthetic cryptand compounds have been known as agents effective in the extraction of ions (as mentioned in Michio Hiraoka: "Crown Compounds", p 176, Kodansha Press, Apr. 1, 1978, for example). While these compounds have high affinity for specific alkali metal ions, they exhibit very poor affinity or virtually no affinity for alkaline earth metal ions such as calcium ions and barium ions.
Transfer of an ion species from an aqueous phase (A) containing the ion species to another aqueous phase (B) has heretofore been attained by the method of interposing between the two aqueous phases an organic phase having the aforementioned known ion extracting agent dissolved therein. In accordance with this conventional method, the amount of ions to be transferred into the aqueous phase (B) is small because the ion-extraction capacity of the agent exhibited in the interface between the aqueous phase (A) and the organic phase is very large as compared with the ion-dissociation capacity of the agent in the interface between the organic phase and the aqueous phase (B). This conventional ion extracting agent has the disadvantage that it is not useful as an agent for the separation and recovery of ions because it permits no easy desorption of ions.
The inventor continued a diligent study directed to development of an ion extracting/separating agent high in affinity for alkaline earth metal ions such as calcium ions and barium ions, namely biologically essential metal ions, and excellent in ability to bind such ions and form a complex, extract the ions effectively, and permit ready dissociation of extracted ions. They have consequently found that a novel cyclic octapeptide formed of eight amino acid residues perfectly answers the aforementioned description of the agent. This invention has been perfected based on this discovery.